Influences of Aeronautical Engineering at Motorsport Engineering (part 1)

Hello Everybody, 

Everyone that loves Motorsport is amazed about how complex is the aerodynamics of a Formula or Prototype car nowadays. Everything that is contained at bodywork design is there for some aerodynamic purpose, which is (in summary) to increase downforce (negative lift force) and decrease drag force, and normally they are made of carbon fiber (with some parts made with carbon fiber mixed with aluminum honeycomb). 

Fig. 1 - Modern F1 car (2022 concept) at Wind Tunnel testing

All of these development came from the interaction of Aeronautical Engineering with Motorsport Engineering, when Motorsport Engineers started to figure out the gains that they could obtain into the car performance with the help of aerodynamics. Even if there were airplanes flying at the world since 1903 and the technologies of aerodynamics being very well studied since them, did you know that aerodynamics just started to play a role in Motorsport just at the end of 1960's and its methodologies and precisions just started to play a strong and decisive impact at car performance at the end of 1970's? Did you know that carbon fiber was just used with success at the beggining of 1980's? Why does that happened so late, even if the aeronautical industry had these technologies very developed and consistent decades earlier? Before entering into the whole history of aerodynamics, let's explain some principles of aerodynamics.

The principles of aerodynamics of a wing starts with the two dimensional flow over an airfoil. At an airfoil, because of its geometry + angle of attack with respect to flow direction, the upper part of the flow faces an nozzle effect higher than the lower part of the flow. Because of that, the local speed of the flow increases at the upper part of the airfoil more than the lower part, generating a lower pressure at the upper part of the airfoil with respect to the lower part. This differential in pressure generates the Lift. If you put the airfoil upside down, this airfoil will generate Downforce. Because of shear forces generated by the flow over the airfoil surface, there is another force resulting: the drag force.

Fig. 2 - Lift and Drag Generation at an airfoil 

The Lift/Downforce and the Drag forces increases with the power of airspeed, being the aerodynamic forces more effective at higher speeds, as can be seen by the equation below:

At a finite-wing, at its tip, because you have this difference of pressure between the lower and upper surface, the flow tends to go from the lower surface (higher pressure) to the upper surface (lower pressure) at the lateral of the wing. This generates a loss of Lift/Downforce and increase of Drag force by the formation of wing-tip vortices (called induced Drag or Drag-due-to-Lift). This wing-tip vortices can be diminished by increasing wing aspect ratio or to use a device at wing tip that diminishes the capability of the flow to go from the lower to upper part of the wing.

Fig. 3 - Wing tip vortices explanation, with a wing tip device to diminish the vortex 

Incresing downforce, the car can increase the normal force at each tyre without increasing weight, leading to maximum values of lateral tyre forces without sliding. That is a very interesting capability because the tires, when cornering, needs to counterreact the centrifugal forces of the car. Also, whithout increasing weight, the centrifugal forces does not increases unless you increase cornering speed. So, with downforce, the car can generate more lateral force to support even higher speeds at cornering (higher lateral accelerations). This means A LOT of gain at cornering speed and performance!

Fig 4 - Increase of maximum lateral force at a tire due to increase of vertical load (red plot) on it

Fig. 5 - Increase of Lateral acceleration because of downforce (in red) in comparison with the increase without downforce (blue). See how great it was when aerodynamics developed in a car.

Fig. 6 - Free body diagram of car when cornering.

Decreasing drag, the car can accelerate much faster into the straights, which means higher accelerations and top speeds (longitudinal performance gain). Of course that increasing downforce and decreasing drag is a compromise solution of aerodynamics because, in most of the cases, increase downforce means increase drag. 

Fig. 7 - Drag polar coefficient of a wing. Take a look that, whenever we increase Lift coefficient, Drag coefficient increases also. Then, to find the optimum Lift/Drag ratio depends of your goals (find a wing incidence that optimizes your goal).

Well, after explaining those aerodynamics principles and its gains on motorsport, let's talk about history. The "aerodynamics invasion" started at 1966, when Jim Hall introduced to the world the Chaparral 2E. This Can-Am car had many innovations, its nose generates downforce, it had lateral sidepods for radiators and a great rear wing which had variable incidence (adjusted by an additional pedal) which had zero angle of attack on straights and a negative angle of attack at the corners (Yes! The first wing car had DRS technology!). With this nose and rear wing, the car could generate more normal force at corners without increasing Mass and Inertia, which boosted the corner capability.

Fig. 8 - Above, the Famous Chaparral 2E 

Jim Hall became so obsessed by generating downforce that, in 1969, he developed the most extreme concept of downforce in a car. The Chaparral 2J took this concept at its state of art. The idea of the car was to generate a pressure close to vaccuum at the floor of the car mechanically, with the use of an air compressor that succioned the air below the floor of the car. To this system to be reliable, it needed perfect sealing. Then, he developed a technology called sliding skirts, which were elastomer that sealed the gap between the floor of the car and the track. A genius effort! The difference of pressure between the lower and the upper part of the car was so big that the downforce was huge! The car so much better than its rivals that it was banned! Yes, it was banned by being so good! But keep this concept in mind, it will be used once again...

Fig. 9 - Chaparral 2J and its air compressors on the back of the car. Note the sliding skirts just below the floor of the car.

When Colin Chapman spotted the opportunity to use wings, he inserted a front and a rear wing them into the Lotus 49 car, creating the Lotus 49B of 1968, which dominated the 1968 season and created the wing era at Formula One. It did not have the variable wing incidence neither lateral sidepods, but, the gains into cornering speed were notable. I was wondering what does Colin Chapman (as an Aeronautical Engineer) thought when he saw the Chaparral 2E with a wing mounted on it. Maybe it was something like: "Why I did not think about this before?!?!" 

Fig. 10 - Lotus 49B. First Formula car with wings. 

From 1968 to 1977, the aerodynamcs played a role in motorsport. This period was when the garagists and factory teams started to enter into aeronautical world, needing aeronautical materials and aeronautical methods, machining and assembly precision in order to develop a competitive car. Wind tunnel tests were highly used to develop proper aerodynamics and the lighter aeronautical materials were used to make the car stronger and lighter. The Copersucar-Fittipaldi team had a large assistant of EMBRAER (brazilian Airplane manufacturer) to develop its aerodynamics, to develop lighter structures and even to instrumentation with put embedded computer to gather data of the car on track (yes, this technology comes also from aeronautical engineering). Moreover, aerodynamics was not the most important factor into the race car performance (you can see during this epoque that the cars had many different layouts of aerodynamics with few difference of performance between them, which means that the "right path" to aerodynamics was not found). It was like that until Colin Chapman (now using all his knowledge acquired in Aeronautical Engineering) developed the Lotus 78 at 1977, the first Ground-effect car. 

Fig. 11 - Lotus 78 (1977), the first Ground Effect car

Ground effect was very known into Aeronautical Engineering and very studied, because its effects of increasing Lift (and decreasing Induced Drag) of the wing when it is close to the ground (by generating an extra barrier against the flow from the lower part of the wing to go to upper part of the wing, as explained earlier). Ground effect benefits the aircraft at Takeoff phase but can be a problem when the aircraft is going to land causing too much flotation into landing when the speed is too high (remember: Lift is proportional to the power of Speed. Then, the landing speed must be very well determined in order to not make the airplane to float too much and it generates difficulties to pilot to land). 

Fig. 12 - Ground Effect on an aircraft 

At World War II (at 40's), the B-29 aircraft used to travel just few meters above the water to increase its range (because ground effect diminishes induced drag). The URSS created even giant flying boats which only flotates over water (carrying more weight than an regular aircraft and travelling faster than a regular ship), called "Ekranoplanes". These Ekranoplanes exists since the 60's. 

Fig. 13 - The Ekranoplan Lun - the biggest and most known Ekranoplan of the World 

But Ground effect just played a role in 1977 at Formula One. Why?! Because nobody realized how to generate ground effect on a car until Colin Chapman. The idea was just fantastic: to create lateral wings with very short span at the sidepods of the car, close to the ground. But wait... very short wing span leads to strong wing tip vortices and a very degraded lift generation (very low aspect ratio wing)... how it is possible to neutralize this wing tip vortices?!?! The answer was simple: to phisically block those vortices by huge endplates, which these endplates with a device that is in contact with the ground, sliding on it when the car is moving (skirts), to make a perfect seal of this wing (and to have almost none wing tip vortices). That was brilliant! Now the cars had sidepods which does not only generates cooling system, but also a lot of downforce! 

Fig. 14 - Lotus 78 with the details of its lateral wing and endplate (Pictute by Giorgio Piola)

Fig. 15 - Downforce coefficient due to skirt sealing gap. Note that the higher the gap, the lower is the Downforce. That is why sliding skirts were developed on that epoch.

This was the time that aerodynanics really started to make the difference. The gains of downforce were MASSIVE with ground effect. After Lotus 78, the fight to find even more downforce played a big role in motorsport and the further developments I will discuss at next post, and this left to the need to use even more strong materials! Wait a little bit and I will show you! :)